Tuesday, March 31, 2009

Understanding of Genetic Variation

Evolution requires genetic variation. If there were no dark moths, the population could not have evolved from mostly light to mostly dark. In order for continuing evolution there must be mechanisms to increase or create genetic variation and mechanisms to decrease it. Mutation is a change in a gene. These changes are the source of new genetic variation. Natural selection operates on this variation.

Genetic variation has two components:
allelic diversity and non-random associations of alleles. Alleles are different versions of the same gene. For example, humans can have A, B, or O alleles that determine one aspect of their blood type. Most animals, including humans, are diploid-they contain two alleles for every gene at every locus, one inherited from their mother and one inherited from their father. Locus is the location of a gene on a chromosome. Humans can be AA, AB, AO, BB, 130, or 00 at the blood group locus. If the two alleles at a locus are of the same type (for instance two A alleles) the individual would be called homozygous. An individual with two different alleles at a locus (for example, an AB individual) is called heterozygous. At any locus there can be many different alleles in a population, more alleles than any single organism can possess. For example, no single human can have an A, B, and an 0 allele.

Allele frequency: The number of organisms in a population carrying a particular allele of gene determines the allele frequency. In population genetics the allele frequency is usually expressed as decimals. Thus, a frequency of 99% is represented as 0.99 and the 1% frequency would be 0.01, because the total population represents 100% or 1.0. In population genetics these are represented as:
p+q=1, where p - frequency of dominant allele & q - frequency of recessive allele.

Thus, in the above example total frequency is 0.99 + 0.01 = 1.0.

If we know the frequency of one allele (gene), the frequency of the other allele can be determined.

Non-random breeding: In most of the natural population, mating is nonrandom. But there are many structural and behavioral mechanisms that prevent the random mating. In populations where there is no random mating, fewer heterozygotes (an organism that has two different alleles at a locus) are found than would be predicted under random mating. A decrease in heterozygotes can be the result of mate choice, or simply the result of population sub-division. Most organisms have a limited dispersal capability, so their mate will be chosen from the local population.

Genetic Drift: The variation in allele frequencies can occur only by chance. This is called genetic drift. Drift is a binomial sampling error of the gene pool. What this means is, the alleles that form the next generation's gene pool are a sample of the alleles from the current generation. When sampled from a population, the frequency of alleles differs slightly due to chance alone. Alleles can increase or decrease in frequency due to drift.

Gene Flow: New organisms may enter a population by migration from another population. If they mate within the population, they can bring new alleles to the local gene pool. This is called gene flow. In some closely related species, fertile hybrids can result from interspecific matings. These hybrids can vector genes from species to species. Gene flow between more distantly related species occurs infrequently. This is called horizontal gene transfer.

Mutation: The cellular machinery that copies DNA sometimes makes mistakes. These mistakes alter the sequence of a gene. This is called a mutation. There are many kinds of mutations. A point mutation is a mutation in which one "letter" of the genetic code is changed to another. Lengths of DNA can also be deleted or inserted in a gene; these are also mutations. Finally, genes or parts of genes can become inverted or duplicated. Typical rates of mutation are between 10-1° and 10-1' mutations per base pair of DNA per generation. Most mutations are thought to be neutral with regards to fitness. Only a small portion of the genome of eukaryotes contains coding segments. And, although some non-coding DNA is involved in gene regulation or other cellular functions, it is probable that most base changes would have no fitness consequence.

Most mutations that have any phenotypic effect are deleterious. Mutations that result in amino acid substitutions can change the shape of a protein, potentially changing or eliminating its function. This can lead to inadequacies in biochemical pathways or interfere with the process of development. Organisms are sufficiently integrated that most random changes will not produce a fitness benefit. Only a very small percentage of mutations are beneficial. The ratio of neutral to deleterious to beneficial mutations is unknown and probably varies with respect to details of the locus in question and environment.

Genetic Load: The existence of disadvantageous alleles in heterozygous genotypes within the population is known as genetic load. The disadvantageous alleles when come as homozygous will affect the organism negatively for their phenotype and their existence. Such organisms may be eliminated from the population when these alleles (if they are deleterious in nature) occur in homozygous condition. If the allele is recessive, its effect won't be seen in any individual until a homozygote is formed. The eventual fate of the allele depends on whether it is neutral, deleterious, or beneficial.

Tags: Bio Technology, Bio Genetics , Genetic Variation

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